Thermosetting powder coating compositions are used extensively to produce durable protective coatings on various materials. Thermosetting coatings, when compared to coatings derived from thermoplastic compositions, generally are tougher, more resistant to solvents and detergents, have better adhesion to metal substrates, and do not soften when exposed to elevated temperatures. However, the curing of thermosetting coatings has created problems in obtaining coatings which have, in addition to the above-stated desirable characteristics, good smoothness and flexibility. Coatings prepared from thermosetting powder compositions, upon the application of heat, may cure or set prior to forming a smooth coating, resulting in a relatively rough or non-uniform finish. Such a coating surface or finish lacks the gloss and luster of coatings typically obtained from thermoplastic compositions. The rough or non-uniform surface problem has caused thermo-setting coatings to be applied from organic solvent systems which are inherently undesirable because of the environmental and safety problems occasioned by the evaporation of the solvent system. Solvent-based coating compositions also suffer from the disadvantage of relatively poor percent utilization, i.e., in some modes of application, only 60 percent or less of the solvent-based coating composition being applied contacts the article or substrate being coated. Thus, a substantial portion of solvent-based coatings can be wasted since that portion which does not contact the article or substrate being coated obviously cannot be reclaimed.
To produce smooth, glossy, uniform coatings, the polymeric materials constituting powder coating compositions must melt within a particular temperature range to permit timely and ample flow of the polymeric material prior to the occurrence of any significant degree of curing, i.e., cross-linking. Powder coating compositions which possess the requisite melting range provide smooth and glossy coatings upon being heated to cure the compositions. In addition to being smooth and glossy, coatings derived from thermosetting coating compositions should exhibit or possess good impact strength, hardness, flexibility, and resistance to solvents and chemicals. For example, good flexibility is essential for powder coating compositions used to coat sheet (coil) steel which is destined to be formed or shaped into articles used in the manufacture of various household appliances and automobiles wherein the sheet metal is flexed or bent at various angles.
It is essential that powder coating compositions remain in a free-flowing, finely divided state for a reasonable period after they are manufactured and packaged. Thus, amorphous polyesters utilized in powder coating formulations desirably possess a glass transition temperature (Tg) higher than the storage temperatures to which the formulations will be exposed. Semi-crystalline polyesters and blends thereof with amorphous polyesters also may be utilized in powder coating formulations. For this application, semi-crystalline polyesters desirably possess a significant degree of crystallinity to prevent caking or sintering of the powder for a reasonable period of time prior to its application to a substrate. Semi-crystalline polyesters used in powder coating formulations also must have melting temperature low enough to permit the compounding of the powder coating formulation without causing the cross-linking agent to react prematurely with the polyesters. The lower melting temperature of the semi-crystalline polyester also is important to achieving good flow of the coating prior to curing and thus aids the production of smooth and glossy coatings.
Finally, the production of tough coatings which are resistant to solvents and chemicals requires adequate cross-linking of the powder coating compositions at curing temperatures and times commonly employed in the industry. In the curing of powder coating compositions, a coated article typically is heated at a temperature in the range of about 325.degree. to 400.degree. F. (163.degree.-204.degree. C.) for up to about 20 minutes causing the coating particles to melt and flow followed by reaction of the cross-linking (curing) agent with the polyester. The degree of curing may be determined by the methyl ethyl ketone rub test described hereinbelow. Normally, a thermosetting coating is considered to be completely or adequately cross-linked if the coating is capable of sustaining 200 double rubs. It is apparent that the use of lower temperatures and/or shorter curing times to produce adequately cross-linked coatings is very advantageous since higher production rates and/or lower energy costs can be achieved thereby.
Powder coating systems based on hydroxyl polyesters and caprolactam-blocked polyisocyanate cross-linking agents have been used extensively in the coatings industry. The most widely used caprolactam-blocked polyisocyanates are those commonly referred to as .epsilon.-caprolactam-blocked isophorone diisocyanate, e.g., those described in U.S. Pat. Nos. 3,822,240, 4,150,211, and 4,212,962. However, the products marketed as .epsilon.-caprolactam-blocked isophorone diisocyanate may consist primarily of the blocked, difunctional, monomeric isophorone diisocyanate, i.e., a mixture of the cis and trans isomers of 3-isocyanatomethyl-3,5,5-trimethylcyclohexylisocyanate, the blocked, difunctional dimer thereof, the blocked, trifunctional trimer thereof or a mixture of the monomeric, dimeric and/or trimeric forms. For example, the blocked polyisocyanate compound used as the cross-linking agent may be a mixture consisting primarily of the .epsilon.-caprolactam-blocked, difunctional, monomeric isophorone diisocyanate and the .epsilon.-caprolactam-blocked, trifunctional trimer of isophorone diisocyanate. The reaction of the isocyanato groups with the blocking compound is reversible at elevated temperatures, e.g., about 150.degree. C. and above, at which temperature the isocyanato groups are available to react with the hydroxyl groups present on the polyester to form urethane linkages, thereby cross-linking or curing the coating composition.
During the curing process using an .epsilon.-caprolactam-blocked polyisocyanate as described above, .epsilon.-caprolactam is liberated from the powder coating compositions. To eliminate the presence of .epsilon.-caprolactam from the workplace, adducts of the 1,3-diazetidine-2,4-dione dimer of isophorone diisocyanate and diols have been developed for use as cross-linking agents in powder coating compositions. Such adducts and powder coating compositions containing the adducts are described in the literature such as, for example, U.S. Pat. No. 4,413,079, German OLS 3,328,133, and the Journal of Chromatography, 472 (1989) 175-195. While these oligomeric cross-linking agents avoid the liberation of .epsilon.-caprolactam, they possess the disadvantage of not being as reactive as the .epsilon.-caprolactam-blocked polyisocyanates when used in combination with commercially-available, amorphous polyesters. Thus, powder coatings based on amorphous polyesters commonly used in the powder coating industry and adducts of the 1,3-diazetidine-2,4-dione dimer of isophorone diisocyanate must be heated at higher temperatures, e.g., 400.degree. F. (204.degree. C.) as compared to 350.degree. F. (177.degree. C.) for caprolactam-blocked isophorone diisocyanate, and/or for longer periods of time to provide adequately cured coatings. However, the use of such higher temperatures does not produce the degree of cross-linking necessary to impart to the cured coating a satisfactory combination of properties, especially resistance to chemicals and solvents.